Referring to FIG. 3, a conventional range-finding system 100 uses triangulation based on reflected external light. Range-finding system 100 is installed in an autofocus camera. A focusing optical system, shown generally at 1, focuses light on a distance-measuring semiconductor integrated circuit 5 (hereinafter IC 5). Focusing optical system 1 includes pair of lenses made up of a right focusing lens 1R and a left focusing lens 1L. Lenses 1R and 1L face a subject T to be imaged. A distance between lenses 1R and 1L produces a parallax. IC 5 includes a right photosensor arrays 2R, upon which light from right lens 1R is imaged, and a left photosensor array 2L upon which light from left lens 1L is imaged. Photosensor arrays convert images of the subject T into analog electrical signals. Right and left quantizing circuits 3R and 3L quantize signals from individual sensors of the photosensor arrays 2R and 2L, respectively. A logic portion 4 calculates a distance signal based on the quantized signals.
Referring now to FIG. 4, the subject T is imaged by the left focusing lens 1L and the right focusing lens 1R. Axes 11 and 12 of left focusing lens 1L and right focusing lens 1R are spaced apart a reference length B. Separate images R and L of the subject T are produced on the photosensor arrays 2R and 2L, respectively, corresponding to a focal plane. The images R and L are inverted real images. Based on the principle of triangulation, or analogy of triangles, the distance d to the subject T is given by EQU d=B.multidot.f.sub.e /(X.sub.1 +X.sub.2)=B.multidot.f.sub.e /X(1)
where: f.sub.e is the distance between the focusing lenses 1R and 1L and the photosensor arrays 2R and 2L, respectively, or the focal distance of the focusing lenses 1R and 1L, X1 and X2 are the distances between the image points P.sub.1 and P.sub.2 of an object point P within the subject and the optical axes 1.sub.1 and 1.sub.2 of the focusing lenses 1L and 1R, respectively.
The sum X of X.sub.1 and X.sub.2 is a relative deviation amount, or the phase difference, between the subject images. The distance d can be found by measuring this spatial phase difference X. Distance-measuring semiconductor integrated circuit 5 uses a function for evaluating the correlation between the subject images, and finds the phase difference X in the form of the number of pitches of the sensors of the photosensors by searching for a zone in which the same subject image is obtained on the photosensor arrays 2R and 2L.
Referring now also to FIG. 5(a), left and right photosensor arrays 2L and 2R each consists of an array of photodiodes D. A photocurrent i flowing through a photodiode D varies in response to the intensity of impinging light. The photocurrent i is integrated in the junction capacitance C of the photodiode D. The integrated potential Q is fed to an input of a threshold circuit COM. Tr is an insulated-gated field-effect transistor which discharges the junction capacitance C in response to a reset signal, thereby resetting the sensor.
Referring now also to FIG. 5(b), when the integrated potential Q exceeds a threshold value V.sub.th, then the output S from the threshold circuit COM is inverted. That is, the intensity of light received by the photodiode D is transformed into a corresponding response time t.sub.s of the output S. As the intensity of the received light increases, the response time t.sub.s shortens.
Referring now to FIG. 6, quantizing circuit 3 in distance-measuring semiconductor integrated circuit 5 includes a clock generator circuit 3a for generating a clock signal CK. Clock signal CK is fed to an input of a counter 3b which may be, for example, an 8-bit counter. Latch portion 3c is equipped with n latch circuits corresponding to the n sensors of the photosensor array 2. The latch circuits latch the total count of the counter 3b while using their respective outputs S.sub.1 -S.sub.n from the sensors as strobe signals. Count values L.sub.1 -L.sub.n, or quantized values, held by the latch circuits, are serially transmitted to an output line O.sub.L1 in response to a selecting signal produced from a decoder 3d. Corrective data C.sub.1 -C.sub.n about variations in sensitivity of the individual photosensors of the photosensor array are stored in a PROM 3e. Corrective data C.sub.1 -C.sub.n is serially transferred to an output line O.sub.L2 in response to a selecting signal from the decoder 3d. A subtracter circuit 3f produces the differences between counted values L.sub.1 -L.sub.n from the sensors and their respective corrective data C.sub.1 -C.sub.n. The differences, representing received light energy, corrected for differences in detector sensitivity, is serially transmitted as N.sub.1 -N.sub.n signals to logic portion 4.
The meanings of the data C.sub.1 -C.sub.n about correction of variations in sensitivity of the sensors are described now. In the above-described range-finding system, the sensitivities of the n sensors vary from each other due to inevitable manufacturing tolerances. The distribution of the variations differs from chip to chip of distance-measuring semiconductor integrated circuit 5. In principle, assuming that a blank white test pattern having no contrast is a subject, a subject image having no contrast should be focused onto the photosensor 2. Any differences in the outputs from sensor to sensor, under this condition, must be due to differences in the sensitivities of the sensors, and not to variations in the scene being imaged. Outputs S.sub.1 -S.sub.n from the sensors all should have the uniform response time t.sub.s. The counted values L.sub.1 -L.sub.n held by the latch circuits of the latch portion 3c should be equal to each other.
In practice, however, variations in the characteristics of the devices forming the sensors due to manufacturing tolerances or errors from variations in net sensitivity of device characteristics of the sensors are superimposed on nonuniformity, or principle error, of the distribution of the amount of light of image focused by the focusing optical system 1. Consequently, even when imaging a blank white scene, the counted values L.sub.1 -L.sub.n differ from each other. More specifically, the intensity of received light, or the illuminance, varies across the image according to the cos.sup.4 in dependence on the displacement of objects from the optical axis on the focal plane (photosensor array 2) of the focusing lens 1. Accordingly, the counted values L.sub.1 -L.sub.n obtained by the latch circuits do not show a uniform distribution due to the cos.sup.4 rule but rather lie on a curve given by 1/ cos.sup.4 (i), as indicated by A in FIG. 7. It is noted that i indicates a sensor address.
Manufacturing errors generally take place at random. The resulting sensitivity variations can be schematically represented by a random curve B shown in FIG. 7. In consequence, the counted values L.sub.1 -L.sub.n lie on a curve C which is obtained by combining curves A and B.
Returning now to FIG. 6, as a technique for correcting the sensitivity variations combined in this way, a correcting circuit (PROM 3e and subtracter circuit 3f) is used. After assembly of the range-finding system, each finished product is tested by imaging the aforementioned blank white test pattern. The counted values L.sub.1 -L.sub.n produced from the chips are written to the PROM 3e by a PROM writer as corrective data C.sub.1 -C.sub.n.
In operation for measuring the distance, the actually measured values L.sub.1 -L.sub.n are obtained from focused images of the subject having contrast. Subtracter circuit 3f produces the differences between the L.sub.1 -L.sub.n and the corrective data C.sub.1 -C.sub.n. The resulting difference values are serially transmitted as corrected counted values N.sub.1 -N.sub.n from the quantizing circuit 3 to the logic portion 4.
The above-described correcting circuit has the following problems. If the resolution at which the distance is measured is improved, then the number of sensor cells of the photosensor array 2, must be increased in order to accommodate the smaller scene increment being sensed by each sensor cell. This increases the number of bits per item of the corrective data C.sub.1 -C.sub.n and the number of data items. For example, in the case of the photosensor 2 having 178 sensors, a storage space of 4 bits is needed per centimeter today. A right sensor array 2R and a left sensor array 2L are needed per chip. Therefore, the area occupied by the PROM 3e is large. Large chip areas are well-known causes of decreases in the manufacturing yield in a semiconductor fabrication process.